1. Field of the Invention
The present invention relates to an electric power circuit protecting apparatus which uses a superconductor for fault current limiting in an electric power system, and more particularly, to an electric power circuit protecting apparatus using a superconductor which is appropriately used in an environment needing a re-closing function of a circuit breaker within a predetermined time from a trip operation of the circuit breaker.
2. Description of the Background Art
In general, in an electric power system, a fault current limiter (FCL) serves to limit mechanical, thermal and electrical stress applied to a bus bar, an insulator and a circuit breaker, by reducing an electric current in generation of a fault current such as an electric shortage current. Therefore, there are increasing demands for the FCL which can limit the large fault current resulting from large electric power energy consumption in the electric power system. However, the development of the superconductor FCL applicable to the electric power system has been delayed due to technical difficulties and high cost.
With the development of a high temperature superconductor, it has been worth noticing of developing an superconductor FCL using a nonlinear voltage-current characteristic of the superconductor. In addition, a high temperature superconductor FCL using liquid nitrogen as a coolant has been developed since 1987.
When the fault current is generated in the electric power system, the superconductor connected to the electric power system is transited from a superconducting state which does not have an electric resistance to a resistor which has a high electric resistance in a short time, which is called a quench characteristic. The high temperature superconductor FCL uses this quench characteristics of the superconductor. Suggested are various types of superconductor FCLs, such as a resistor type FCL, an inductive type FCL and a hybrid type FCL.
On the other hand, in the countries in which many lines for power transmission or power distribution have been installed on the ground like Korea and Japan, when an accidental error or an one-time error occurs by exposure of the lines, in order to rapidly resume electric power supply, the circuit breaker is requested to be automatically re-closed after a predetermined time, for example, 0.3 to 1 second from circuit opening, namely, a trip operation. It is difficult to use the superconductor element as the FCL in the environment requiring the re-closing function of the circuit breaker within a predetermined time from the trip operation of the circuit breaker. In case the superconductor element is quenched from the superconducting state to the resistance state due to the fault current such as the electric shortage current on the lines for power transmission or power distribution (hereinafter, referred to as ‘circuit’), it takes a much longer time than 1 second to supplement the coolant evaporated from the circumference of the superconductor element and cool the superconductor element in order to transit the resistance state to the superconducting state.